The present invention relates to drying sections of paper making machines and, more particularly, to substitutes for and/or improvements of vacuum rolls and/or felt rolls used in drying sections.
As is well known, in a single tier dryer section the drying cylinders are arranged in a line following one another. In contrast, in a double-tier dryer section these cylinders are stacked in two tiers--two lines--, the paper web meandering alternatingly between the two tiers, in a zigzag pattern.
Although single tier dryer sections were commercialized in a meaningful way only relatively recently in the early eighties, they have been described in the literature for at least a half a century. As to double tier dryers, those have been in actual use for well over a century. Accordingly, to avoid needless description of well known technology, reference is made to the following United State patents which mostly describe single tier drying sections: U.S. Pat. Nos. 1,656,853; 2,537,129; 3,448,529; 3,868,780; 4,359,827; 4,427,736; 4,483,083; 4,677,762; 4,807,371; 4,850,121; 4,876,803; 4,882,854; 4,972,608; 4,974,340; 4,980,979; 4,982,513; 5,101,577; 5,105,501; 5,135,614; 5,144,758; 5,146,696; 5,269,074; and 5,279,049. The contents of each of the above patents is incorporated by reference herein.
Common to both single and double tier drying sections, the drying cylinders are divided into groups, each group having an associated felt or fabric, in well known manner. Of relevance here is the fact that in a double tier dryer section the felt is guided from one drying cylinder the next by so-called felt rolls. Similarly, in modern single tier drying sections, the same guiding function is carried out by so-called vacuum rolls, which also go by other names, e.g. suction rolls, suction felt rolls, etc.
With the passage of time, the operational speeds and the widths of paper machines have steadily increased. For example, nowadays finished paper measuring about 400 inches in width spews out of these paper machines at speeds well in excess of 4,000 feet per minute (about 45 miles per hour) and often even faster. In other words, the machine produces in one minute a sheet of paper that is 400 inches wide and almost a mile long. To produce 400 inch wide paper, the drying cylinders of these machines have to have axial lengths of well over 400 inches. In fact, these drying cylinders are huge, not only in width but also in diameter which typically can be on the order of 6 to 7 feet. These devices weigh many, many tons. Nonetheless, these cylinders are well worth their weight and cost because they provide very large heated surfaces over which the paper web travels as it is produced, and are the very reason which has enabled the realization of very fast and efficient paper making machines.
In contrast, there is no intrinsic overpowering reason for felt or vacuum rolls to be of gigantic size. These latter devices function primarily to transfer the felt, and in the case of vacuum rolls the felt and the web, from one drying cylinder to the next. Still, these devices have to be as long axially as the drying cylinders so as to span the width of the paper machine. Moreover, their diameters must still be quite large to ensure that they will not sag or bow at their centers, which would be extremely detrimental to the reliability and runability of the paper machine. In any event, current thinking in the art is that the ever increasing machine speeds encountered today make it useful to support the paper web substantially throughout its journey through the drying section, certainly during the first few dryer sections of the drying section and preferably throughout. In this regard, paper makers speak of the need to operate their machines without any "open draw," i.e. with the paper web firmly supported on the dryers, or on the felts when the web travels between dryers, typically through the use of intermediate vacuum rolls. Therefore, many drying section designers are compelled to use expensive, massive vacuum rolls and/or very long and large felt rolls which substantially increases machine costs and machine operation expenditures.